I believe that the lead producers at least do play the game to test it out (as part of their work hours). I've seen both Grant and Ninja make many posts about it. I have no idea what that entails, but I can say that they do at least sometimes test with other packs, because there have been screenshots and references. A few devs play somewhat regularly, according to their claims.
The problem lies in that there are many different play styles, and devoted players of the game most likely play for many more hours during the week than any of the devs. We have players who don't work, who are retired, disabled, ill, etc., plus those who spend a healthy chunk of their free time playing the game, so more content is covered on a more regular basis. So a dev's test run is nowhere near the equivalent hours put in. Plus, the sheer volume of millions of players are much more likely to find issues than the 200 or so people developing the game, with only a fraction of that number actually playing it.
I think that the idea of having confidential play testers is a good one. They could further utilize the Game Changer program to do this, because they really do need dedicated long-term players to deep dive into the content to find if it integrates well.
But it's also helpful for players to meet halfway and try to understand that sometimes design decisions have to be made that just can't please everyone. It's unfortunate, but it's just the way things are. Maybe there will be more flexibility with Sims 5's design, who knows?
